1. Field of the Invention
Aspects of the present invention relate to a polymer electrolyte membrane fuel cell (PEMFC), and more particularly, to a PEMFC having an increased current density and an improved temperature distribution.
2. Description of the Related Art
A fuel cell is an electric generator that converts chemical energy of a fuel into electrical energy, through a chemical reaction. A fuel cell can continuously generate electricity, as long as fuel is supplied.
A polymer electrolyte membrane fuel cell (PEMFC) includes a fuel cell stack, in which unit cells are stacked in series. Each unit cell includes: a cathode and an anode, which are respectively formed on either side of an electrolyte membrane; and two plates having flow channels to supply an oxidant (oxygen in air) to the cathode, and a hydrogen fuel to the anode. The plate can be a bipolar plate, in which the flow channels are formed on both sides thereof, to respectively supply the air and fuel to the cathode and the anode, which contact the plate.
FIG. 1 is a perspective view of the structure of a conventional unit cell. Referring to FIG. 1, the unit cell includes a membrane electrode assembly (MEA) comprising an electrolyte membrane 2 disposed between an anode 1 and a cathode 3. A pair of bipolar plates 10 are disposed on opposing sides of the MEA. Flow channels 11, through which a hydrogen fuel is supplied to the anode 1, are formed on one side of the bipolar plates 10, and flow channels 12, through which air is supplied to the cathode 2, are formed on an opposing side of the bipolar plates 10. The stacking of such unit cells forms a fuel cell stack.
Each bipolar plate 10 includes a fuel inlet 11a, a fuel outlet 11b, an inlet manifold 13a disposed between the fuel inlet 11a and the flow channel 11, and an outlet manifold 13b disposed between the flow channel 11 and the fuel outlet 11b. Each bipolar plate 10 also includes an oxidant inlet 12a, an oxidant outlet 12b, an inlet manifold 14a disposed between the oxidant inlet 12aand the flow channel 12, and an outlet manifold 14b disposed between the low channel 12 and the fuel outlet 12b. 
A gasket 30 to seal the unit cell is interposed between the bipolar plates 10. The leakage of the fluids from the flow channels 11 and 12 is prevented, by placing the MEA 20 on a central portion of the bipolar plate 10, and then attaching the gasket 30 to edges of the bipolar plates 10.
FIGS. 2A and 2B are cross-sectional views of the manifolds of the first and second bipolar plates 10. As schematically illustrated in FIG. 2A, the gasket 30 is disposed between the manifold 13a and the manifold 14a. Since the gasket 30 is formed of an elastic material, the gasket 30 may block the manifolds 13a, 13b, 14a, and 14b. That is, as illustrated in FIG. 2B, for example, if the gasket 30 is bent towards the manifold 13a, the gasket 30 can block a portion of the manifold 13a and the flow channel 11.
Conventional bipolar plates have a thickness of approximately 1 cm, and are relatively deep. Thus, even if the gasket 30 is slightly bent, the gasket 30 cannot completely block the manifold 13a and the flow channel 11. However, if the thickness of the conventional bipolar plates is reduced to a few mm, the blocking of the manifolds 13a, 13b, 14a, and 14b can occur. In particular, if the electrolyte membrane 2 swells, by absorbing water during a fuel cell reaction, the gasket 30 cannot be rigidly supported, and thus, there is high possibility that the gasket 30 can be pushed into one of the manifolds 13a, 13b, 14a, and 14b, and then stuck on an inner wall of one of the manifolds. In this case, fluids cannot freely flow through the flow channels 11 and 12, resulting in abnormal operation of the fuel cell.
In order to address the problem described above, as depicted in FIG. 3, a metal bridge plate 40 is used to cover the manifolds 13a, 13b, 14a, and 14b. A step 15 is formed in each of the manifolds 13a, 13b, 14a, and 14b, and the metal bridge plate 40 is disposed on the step 15. Then, the gasket 30 is attached to the metal bridge plate 40. In this way, the air-tightness of the manifolds 13a, 13b, 14a, and 14b is maintained, and the blocking of the manifolds 13a, 13b, 14a, and 14b, due to the gasket 30, is prevented. However, the size of the manifolds 13a, 13b, 14a, and 14b is reduced by the metal bridge plate 40, resulting in flow restrictions. Also, the unit cells become more complicated, and the metal bridge plate 40 may corrode over time.
In a PEMFC having the conventional bipolar plate 10, if impurities are present in the hydrogen fuel and/or the oxidant, the concentration of hydrogen and oxidant is reduced along the flow paths, and as a result, the current density is reduced along the flow paths. Also, heat flux generated in the MEA is related to the current density, and thus, the temperature can decrease along the flow paths. In particular, a large amount of CO2 or CO can be mixed into the hydrogen fuel supplied to a PEMFC that is operated at a high temperature, and air can be used as the oxidant, and thus, the current density and thermal density can be non-uniform.